Cardiac pacemaker with regulated power supply



United States Patent CARDIAC PACEMAKER WITH REGULATED POWER SUPPLY 3 Claims, 1 Drawing Fig.

Primary Examiner-William E. Kamm Attorney-Lew Schwartz ABSTRACT: A cardiac pacer, which may be a demand pacer, with a calibrated stimulating pulse rate which is preferably adjustable. The pacer utilizes a low current drain, regulated power supply which provides a current into a timing circuit, the current being substantially independent of supply voltage [52] US. Cl 128/421; over a large range f l voltage Variations;

320/391323/66 The regulated power supply includes a series circuit con- [51 Int. Cl A6ln 1/36 raining a source f voltage, a fi |d eff t device f producing [50] Fleld ofseal'ch 128/419(P a Constant current, and an impedance for providing a Substam D1ge$t)'4|9"4221 523/65! 683 tially constant voltage across it when a substantially constant 320/39 current is applied to it. A transistor is connected across the constant voltage producing'impedance so that it exhibits a [56] References cued high input impedance and provides a current, in its collector UNITED STATES PATENTS circuit, which varies substantially independently of the source 3,109,430 1 1/1963 Tischler 128/422 voltage. In the preferred form, a variable resistor in the 3,1 14,872 12/1963 Allard 323/22X transistor emitter circuit makes the magnitude of the collector 3,303,413 2/1967 Warner. Jr. et al. 323/22X current adjustable over a predetermined range.

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ROBERT K. ANDERSON ATTORNEYS CARDIAC PACEMAKER WITH REGULATED POWER SUPPLY BACKGROUND OF THE INVENTION This invention relates to electronic circuits, and more particularly to cardiac pacer circuits adapted to supply heart stimulating pulses to a patient on demand.

Cardiac pacers are well known, as are demand pacers which supply heart stimulating pulses only when the patient's heart skips one or more beats. Such demand I pacers are often designed to respond to the R-wave produced by the patient's heart. Prior art pacers have been subject to the disadvantage, however, that the timing between pulses is dependent upon the supply voltage. That disadvantage is not great if the pacer is adapted to be powered from line current, but such devices are not portable with a mobile patient. j

Portable pacer units-should be usable over an extended period of time, preferably several years, without replacement of the batteries. For that reason, most portable pacers, both.

external and implantable, have been designed for low power drain. Even those devices were not completely acceptable because their pulse rate gradually changes as battery voltage deteriorates. An additional problem was created when it was desired to make an adjustable rate device with a rate calibration. As the battery voltage dropped due to continued use, the calibration, if in number of beats per minute, becomes erroneous. Zener diodes are often utilized to regulate supply voltage. However, those devices are not usable in low power drain pacers. Zener diodes draw a current which is at least an order of magnitude greater than the entire current drain desired in a portable battery powered pacer.

Field effect devices have also been used to provide a constant .current source which is relatively independent of variations in the supply voltage. The difficulty with using such devices is that the actual current obtainable with the same gate bias varies as much as 20 percent from device to device. Thus, when it is desired to make a constant current of controlable magnitude, it is difficult, if not impossible, to obtain accurate variable resistors capable l controlling the current, which may vary as much as 20 20 from one device to the next, within a specified range for all the pacers.

SUMMARY OF THE INVENTION This invention is intended to overcome theaforementioned difficulties of prior art-devices. The invention provides a low power drain pacer having a regulated power supply which drives a pulse generator at a rate which is substantially independent of the supply voltage over a wide range of such voltages. The power supply preferably includes a rate control, calibrated in beats per minute, which changes less than I percent even though supply voltage decreases by as much as 20 percent.

The invention provides a regulated power supply including a series circuit including a source of voltage, means for providing a substantially constant current independent of relatively wide variations in voltagethereacross, and a means for providing a substantially constant voltage when a substantially con- 7 stant current is applied to it.'The power supply further inpulse generator timer to fire the next pulse the normal interval after the natural heartbeat. The pulse generator of such a device is set to 20 at a slightly slower rate than the natural heartbeat rate. Therefore, the demand pacer is continually reset and provides no pulse output so long as the heart beats naturally. Should the heart skip a beat, however, the pacer inserts that beat in the normal sequence. As long as the heart fails to provide natural beats in the normal sequence, the demand pacer stimulates beats at the pacer rate. The preferred form of the invention provides an adjustable rate, calibrated in beats per minute in which the rate is substantially unaffected by supply voltage variations of as much as 20 percent or by the stimulated heartbeat which immediately follows each pacer pulse.

BRIEF DESCRIPTION OF THE DRAWING The single FIG. is a schematic circuit diagram of a cardiac demand pacer utilizing the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The single FIG. discloses a cardiac demand pacer including a regulated power supply 10 which provides power for a pulse generator 11. Pulses from generator 11 are fed to a voltage increasing circuit 12 which applies an increases output voltage across a pair of electrodes 13 and 14. Electrodes l3 and 14 are adapted to contact a patient so as to deliverheart stimulating pulses to the cardiac tissue. One or more of electrodes 13 and 14 may be designed to come directly in contact with cardiac tissue. The demand portion of the pacer includes an R-wave amplifier 15 which is connected to receive andamplify R- wave signals from electrodes 13 and 14 andto produce a signal output for a sensed R-wave of either polarity. The output signal from amplifier 15 is coupled to a refractory circuit 16 which is designed to receive a signal from amplifier 15, ac-

. tivate a reset mechanism 17, and prevent further signals pendent of source voltage variations when a relatively constant voltage is connected between the input terminals. The constant voltage means is connected between the input terminals of the current control means. In the. preferred structure, the current control means includes means for varying the output current.

The invention further provides a cardiac pacer having a regulated power supply, above described, connected to provide a current into the timing circuit of a pulse generator. The pacer also has electrodes for applying pulses from the pulse generator to the patients heart. The pacer is preferably a demand device which also includes circuitry for sensing natural heartbeats intervening between pulses and for resetting the received from amplifier 15 within a predetermined time period from reactivating reset mechanism 17. Thus, refractory circuit 16 makes reset mechanism 17 insensitive to signals sensed by amplifier 15 within a predetermined time interval after a first signal is sensed. Reset mechanism 17 is connected to reset a timing mechanism within pulse generator 11 to a predetermined level in synchronism with the signal from amplifier 15. Thus, the demand pacer provides a stimulating pulse if, and only if, a natural heartbeat does not occur within a predetermined time after the last reset signal, whether that signal was caused by a pacer pulse or by a prior natural heartbeat.

Power supply 10 includes a source of voltage, here shown as a battery 20, with a capacitor 21 connected between the positive and negative terminals. Capacitor 21 extends battery life by reducing the effects of high current drain during the pacer pulse. The positive terminal of battery 20 is connected to a supply voltage conductor 18 through a pushbutton, on-off switch 22. The negative terminal of battery 20 is connected to a ground conductor 19. A device which provides a substantially constant current independent of relatively wide variations in the voltage across it, here shown as a field effect device 23 having a source electrode 24, a drain electrode 25, and a gate electrode 26 is provided with a resistor 27 connected between source electrode 24 and ground line 19. Gate electrode 26 is directly connected to ground line 19. Connected in series with the source-drain circuit of field effect device 23 is means for providing a substantially constant voltage when a substantially constant current is applied to it. That means, as here shown, includes a resistor 28 and a variable resistor 30. One end of resistor 28 is connected to drain electrode 25 and the other end is connected tothe cathode of a Power supply-10 alsot'includes a current control means hav- 'ing'a pair of inputterminals and a pair of output terminals, exhibitingv high impedance between the input terminals and providin g'a currentbetween the output terminals which varies intermediate resistor 67- and capacitor 68 to the opposite plate 5 of capacitor 70.

1 The opposite plate of capaci r zfl is also connected to one plate of a lowfrequency shapiri capacitor 73,-whose opposite substantially independently of source voltage variations when plate is connected to the gate el'ectr'odeof a field effect device 6 a relatively constant voltage is applied across the input'ter- 74. The gate electrode of field effect-device 74 is connected 7 "Fi lere s own =a1 arl i t r 31 and variable resistors 32 through a biasingiresistor 75. to ground conductor 19 and is an On f th inpu te m i'i v f th r en n r also connected to the drain electrode of field effect device 74 I nie i pr y mm h n of its e pi tterminals Y through-ahigh frequericy rolloff capacitor 76. A resistor 77 is S sh w at jllhcliQh T a ehtmn 1 8 the other qtcon nected -between supply voltage conductor 18 and the drain inpu e na and I O 9 ansi e is the other electrode of field effect de vice.74. A- resistor 78 is connected o'iitput terminal; Transist0r'3l has t fi 'Q between the source electrode of field effect device 74 -and nec't drain electrode -a gfi emitter fi ti groundconductor 19. A low frequency shaping capacitor? is CCEQ n n b resistor -N b resistor. 1 also connected between the source electrode. of'field effect has its opposite end connected to one end of asecond variable d i 74 d ground conductor 19. FS w i QPP and Wiper connected to A low frequency shaping capacitor 80 is connected between pp y- Cm ls at jun ctiori 5l. Regulated power the tdrainelectrode offield effect device 74 and theigate elecu p y Provides,v at e l e o of transistor 31, a-Currem trode' of a field effect device 81-, A pair of opposed diodes s2 Wh s magnitude u r varying the impedance of and 83 are connected in-parallel between the gate electrode of 1 sisttof Resistor 32 is preteetedfm'm fixtraneous magnetic field effect device 81 and ground conductor 19. A resistor 84 e d by iii-Shielding 34 e-Fw a conductor which in and a highr frequency rolloff capacitor 86 are connected in i is directly connected t ground cQndu-ctol' 1 arallel between supply voltage conductor 18 and the drain W Pu ra rthe collecfor of electrode offield effect device 81. A resistor 85 and a low Fa l ected to one plate of a timing flPaU Q 36 ;-.25 frequency shaping capacitor 87 are connected in parallel mi i 'P connect-ed to'g'mund conductor w. betweenthe source electrode of f eld effect device 81 and The collector of transistor 31 is also connected to one end of a ground conductortlq v I s o whose opPosit-e end is connected to the base of a I A low frequency shaping capacitor 88 is connected between Fansismnaa The'9pposite end of resistor is also connected the drain electrode of field effect device81 and a base elec- 9 .theflathode Ora .anode is connFcted 30 .trode of a transistor '89; A biasing resistor 90 is connected F -Q Almgh 'i bypass capac'tor t 7 between thebas fof transistor 89 andground cOnductory19. @m bcwee the'basev P P 38 and ground r Anotherbiasing resistor 91 isconnected between supply volt- 9 The base 0f F lsconne'cted aground ,ageconductonlSandthe base of transistor 89. The emitter of through 'resltsmrlu, find a tempera-m -transistor:89isconnected through the parallel combination of 1' pwsatmg mode A diode: has anode conngpltetd tome a resistor 92 and a capacitor 94 to supply voltage conductor of translsmr 31 and cathode connected m the .18. The collector of transistor 89 is connected through the leciorvof t-tansisvwtas' A amen; limiting-faggot connects parallel combination of a capacitor 95 and a resistor 93 to supply,voltageconductoril8 tothe collector of transistorz38. groun conducto Collector of transistor? connected through a 'The collector of transistor- 89 is also connected to one plate sister '45: to thebase of a transistor 46. The emitter of ofa low frequehcy shaping capacitor is dire-my copnected o supply'voltage Conductor In therefractory circuit, the other Plate of capacitor 96 is j 18 Thg-,colleCtor of trans stor46 is connected througha load connected through arresistor 99 to the base of a transistor 1100 12223 fz zg g g i fi gi z giii The emitter of transistor 100 is directly connected to supply -;a rfdacap;cito:'50 to the base of transistor38 The output of 4 Voltage conductorls' cqlector translstorloo con- 7 nected through a -cu rrent limiting resistor 10] to ground conpulse generator ll appears at thecollector of transistor ductor A dibde. lozlhas its cathode connected to Supply iInyol-tageinqeasing circuit the collector of transistor volta e conductor and its anode connected to the base of is'di'regflyconnected to base of'a transistor The coltrarisi stor 100 The base of transistor-1 00 is also connected to lectoi' of transistor 53 is directly connected to supply voltage 6 supplyiyoltage conductor 18 through a resistor 103. f i f gz g g g a a 2 Thecollector of transistor-100 is also connected through a m ting resistor: 'to-groun con uctor e emitter o i e transistor53 is also connected directly to electrode 13.The 'i g m' q Y32 9 d g of L 9 base of transistor 53 is connected through a resistor to the .f- 2 trans?!- d 3 2: fi a f gf base. of -a transistor 56. T he t mitter of transistor 56 iscone o groun con F ec m 0 transls'or ncted through avyariable kesisltbrt's-l and a resismrmsslto v 1 06 is connected to the base of. transistor 1 00 through arengfidund conductor 9. supply gg conductor 18 is com -S1St0l'108. The yernitter of transistor106 is directly connegted I nected through aresistort59anda diode to thel ll m {0 ground conductor-r19. A transistor 109 has its collector d trnsiStor-dSfiIhe Collector of vtrarisistor-56 is further conemmer connegtedfactoss the plat? capacuor 104 and m ha plate is connected to electrode 14, An interference shield62 IS it I Y g disposed about variable resistor 51. to screen out unwanted exa and 3 tqywpd xydt-emallfieldseshield 62 iscohnccted to qonquctor conductor -19 The output from refractory circu t 16 is taken In R-wave amplifier 1 5, electrode 14 isconnected to ground g 3 m g g fl 11 14 h h h q ugh a variable voltage-divider which includes a resistor. l 5 beset Z S P F a set transistor w ic i as E and a resistor .65 which carries asmovable wiper. A high 35c copnfected'to s mtermtedwfte MW fre qu ency bypass capacitor 66 is-connected between resistor The emmer dlrecny'connectcdto 64 and ground conductorllgz The wiper resistor hasits groundmonductor l9 and its collector is connected to theooL freeerid connected to one end of a resistor 67. The opposite o of trans'stor'a end ofresistor 167 is connected through fl CflpUCilOl' 68 to .OPERATlON when switchiZZ is closed, a circuit is completed from the positive terminal ot battery 20 through resistor 3 0, diode 2 9, i resistor 28, the-draimsource circuit of field effect; evic'e :23

groundgconductofli flhe wiper of resistor 65 is also con- 'nected to oneplateofiacapa citor 69 whose opposite plate is cqrinected to one plate of the capacitor 70. Aresistor'7l is nected fronj apo int intermediate capacitors 69 and 70 to field effect device 23 is connected in the manner shown, the source-drain current is substantially independent of the source-drain voltage over a wide range of voltages. However, the particular current value obtainable in the source-drain circuit varies greatly from device to device. For that reason, either resistor 27 or resistor 28 can be chose as a select" resistor which is experimentally chosen in each circuit to provide a source-drain current of a predetermined level. Final trim of the current through the circuit is done by adjustment of trim resistor 30. Diode 29, while desirable when the pacer is to be used over a relatively wide range of ambient temperatures, is not required. Diode 29 provides temperature compensation for the variation in emitter junction characteristics of transistor 31 with temperature variation. Since a predetermined magnitude constant current flows through the sourcedrain circuit of field effect device 23, the voltage across resistor 30, diode 29 and resistor 28 is constant so long as sub stantially all of the field effect device 23 source-drain current, and no other, flows through those components. A high input impedance device is presented between the base of transistor 31 and junction 51 so no appreciable current flows into field effect device 23 from that circuit branch. The base bias on transistor 31 is therefore constant regardless of supply voltage variations, and its collector current is controlled by the resistance in the emitter circuit. The magnitude of the current in the collectorcircuit of transistor 31 is controlled externally by variable resistor 32 whose wiper is connected to a dial on the case of the cardiac pacer. That dial is calibrated in beats per minute in ranges from about 50 beats per minute to about 150 beats per minute. Variable resistor 33 is used for final trim to calibrate the unit before it is put into use.

Current flowing from the collector of transistor 31 into pulse generator 11 charges timing capacitor 36 to provide the timing between pulses. Resistor 37 is a high valued resistor which allows little current to leak from capacitor 36 during the charging cycle. After a predetermined charge time, the voltage across resistor 41 becomes large enough to forward bias the base of transistor 38. When transistor 38 starts to turn on, its collector potential falls toward ground, causing base current to flow through transistor 46 and resistor 45, turning on transistor 46. Turn on of transistor 46 brings its collector nearly to supply voltage and supplies base current for transistor 38 holding it on. Turn on of transistor 38 allows capacitor 36 to dump quickly through diode 43 and the collector emitter circuit of transistor 38. As soon as capacitor 50 becomes fully charged, base current is cut off in transistor 38 and it turns off, thereby turning off transistor 46. When transistors 38 and 46 turn off, capacitor 50 discharges through resistor 49, resistor 47, meter 48 and diode 39. Capacitor 36 is then ready to begin timing for the next pulse. That timing begins immediately and proceeds uninterrupted in a nondemand unit, but is interrupted and reset in a demand unit as will be explained below. Diode 42 provides temperature compensation for the emitter base junction of transistor 38.

The output of pulse generator 11 is taken at the collector of transistor 46. A positive pulse is generated at this point by turnon and turnoff of transistor 46. The interval between pulses is determined by the magnitude of the current flowing in the collector circuit of transistor 31 and the size of timing capacitor 36. The pulse width is determined by the magnitudes of re sistor 49 and capacitor 50. A visual indication of the generation of pulses is obtained from observation of meter 48.

In portable cardiac pacers, it is highly desirable to apply a pulse to the cardiac tissue which is substantially in excess of the supply voltage, which is often about volts. For this reason, pulses from pulse generator 11 are fed into a voltage increasing circuit 12 which is capable of producing pulses with nearly double the supply voltage. Immediately prior to a pulse from pulse generator 11, capacitor 61 is fully charged from positive supply conductor 18 so that approximately 10 volts appears across its plates. Electrodes 13 and 14 are both at ground potential. Appearance of the positive going pulse at the collector of transistor 46 drives transistor 53 into saturation and raises electrode 13 to nearly supply voltage, approximately +10 volts. Resistor 58 is a small valued resistor and resistors 57 and 59 are of approximately equal values when the entire length of resistor 57 is in circuit with the emitter of transistor 56. The positive going pulse also is transmitted through resistor 55 and turns on transistor 56 which brings the collector of transistor 56 rapidly toward ground potential (when resistor 57 is bypassed by its wiper). Since the voltage across capacitor 61 cannot change instantaneously, electrode 14 immediately falls to a level of almost supply voltage below ground potential. Therefore, the potential difference between electrodes 13 and 14 is approximately twice the supply voltage during a pulse.

Variable resistor 57 provides control of the current flowing between electrodes 13 and 14 during the pacer pulse. The wiper of resistor 57 is attached to an external control knob which is calibrated in milliamperes. As can readily be seen, as more resistance is inserted in the emitter circuit of transistor 56, its collector-emitter current is reduced and the voltage difference between electrodes 13 and 14 is likewise reduced because the collector of transistor 56 falls to a point intermediate between supply voltage and ground.

During the pacer pulse, capacitor 61 discharges through the collector-emitter circuit of transistor 56, resistor 57, resistor 58, resistor 54, electrode 13, the patients'body, and electrode 14. Upon termination of the positive pulse from the collector of transistor 46, transistors 53 and 56 turnoff and capacitor 61 begins charging again from supply voltage conductor 18. When capacitor 61 is again fully charged, electrodes 13 and 14 are both at ground potential. It can also be seen that use of capacitor 61 in the output circuit provides a bidirectional current flow between electrodes 13 and 14 which is considered advantageous.

The portion of the pacer apparatus described above will, if used without the remainder of the circuitry, provide free running pulses at a rate chosen by positioning of the wiper on variable resistor 32 and having a magnitude chosen by positioning of the wiper on resistor 57. The remainder of the circuitry pertains to demand pacer which is advantageous because energy is conserved in the battery when the heart is beating of its own accord and because pacer pulses are not supplied when they might compete with natural heartbeats.

R-wave amplifier 15 has a specially designed response curve which make it highly selective. its response curve is shaped so that the amplifier rejects extremely low frequency signals, such as the heart's 'I -wave, rejects 60 hertz interference signals and rejects high frequency interference signals (e.g. 400 hertz). The amplifier sensitivity is relatively low for signals of less than 15 hertz and rapidly rises to peak sensitivity at about 25 hertz. The sensitivity remains high up to nearly 60 hertz and then drops sharply in a notch at 60 hertz and rises again quite sharply to a sensitivity less than that at 25 hertz for frequencies above 60 hertz. The sensitivity slowly decreases above about 100 hertz and the amplifier is substantially insensitive to signals of a frequency greater than 200 hertz.

Signals from the heart and pacer pulses are sensed through electrode 14. These signals are picked off by the wiper of variable resistor 65 and are fed into a twin-T filter network which includes capacitor 68, 69 and 70, and resistors 67, 71 and 72. The twin-T network is designed to provide the notch at 60 hertz for strong rejection of interference at that frequency. The output of the twin-T notch filter is fed into a field effect device 74 to prevent loading the twin- T, thereby making the notch sharper. Capacitors 73, 80, 88 and 96, the network containing resistor 78 and and capacitor 87 provide low frequency shaping to make the demand circuitry insensitive to T-waves generated by the heart and other low frequency signals. The R-C combinations of resistor 78 and capacitor 79 and resistor 85 and capacitor 87 in the source circuits of field effect devices 74 and 81, respectively, provide low frequency shaping because those combinations provide an impedance which decreases with increasing frequency. Therefore, the output sensitivity of field effect devices 74 and 81 increase as the frequency increases. Capacitor 76 contributes to the high fretiuency rolloff in sensitivity.

{Diodes 82 and 83 provide a gate reference for field effect de'iace 81 and further provide signal level limiting at the gate of field effect device 81. The primary purpose of diodes 82 and 83 is to prevent overdriving the amplifier when a pacer pul'sj'eis sensed at electrode 14 and transmitted back through R wave amplifier 15. Pacer pulses are of the order of 15 volts which is 15,000 timesthe one millivolt minimal R-wave which is' required to be sensed. The amplifier must be sensitive enough to sense theR-wave yet must recover rapidly from a pacer. pulse..Diodes 82 and 83 aid in this function by shunting most of the pacer pulse energy directly; to ground conductor 19.?3- 1 'Ihe sensed signal from field effect device 74 turns on field effect device 81, which in turn provides a signal at the base of transistor 89. Transistor 89 is biased on continuously by bias resistors 90 and 91. The purpose of transistor stage 89, re-

' rs 92 and 93 andcapacitors 94 and 95 is to provide additlo'nal gain and to generate a bipolar output'signal in response monopolar signal of either polarity applied at electrode his is necessary because R-waves of either polarity can be generated in the same patient. Resistors 9 2 and 93 are chosen to be of a magnitude which differs by a small factor, substanti lly'less than 10. Capacitor 94 forms a differentiating circuit with" resistor 92. ln such a circuit, a monopolar input signal produces a bipolar output signal wherein the opposite polarity p rtions of the signal each have substantial magnitude.

The signal, then, which is fed into resistor 99 as a result of ap R-wave of pacer pulse at electrode 14 contains both a positive'going and a negative going component. The major difference in the output signal depending on whether the input Signal is positive going-or negative going is which of the posiaye going or negative going components is generated first.

To obtain a demand pacer as above described whose rate is calibrated correctly when pacer pulses are fired into either a purely resistive load or into the heart, arefractory circuit is required. This circuit is needed because a pacer pulse fired intb'the heart elicits a driven R-wave which closely follows the pacer pulse, whereas no such phenomenon occurs when the pacer is fired into a purely resistive load. Therefore, if no precautions are taken, the demand pacer will be reset by the pacer pulse,,will begin timing for the next pacer pulse, and will hesreset again by the driven R-wave which follows that pacer pulse. Therefore, in the absence of a refractory circuit, the pacer will generate pulses at one rate when connected across a resistive load and will generate pulses at a somewhat slower ratewhen connected to an actual heart. Such a response also results in a timg,,interval between the last natural heartbeat and/the first generated pulse which is different than the time interval between two succeeding generated pulses (and therefore that between two succeeding generated heart beats). Refractory circuit 16 is designed to eliminate rate variation dufe to sensing of the driven R-wave by making the reset mechanism insensitive to any signal which follows a first reset signal from amplifier 15 by less than a predetermined time interval.

,The bipolar signal from amplifier 15 is transmitted to the base of transistor 100. The negative going portion of that signal turns on transistor 100 bringing its collector to nearly supply voltage. Turn on of transistor 100 begins charging of capacitor 104 and provides base current for transistor 106 t ough capacitor 104 and resistor 105. Transistor 106'then 11 1 son bringing the voltage at the point intermediate res tors 103 and 108 to a predetermined level between supply voltage and ground, thereby holding transistor 100 on until capacitor 104 is charged. The on time of transistors 100 and 106 is determined by the magnitudes of capacitor 104 and re- 'si'stor 105. As soon as capacitor 104 is fully charged, transistor 106,}turn's off removing base current from transistor 100 and turning it off.

..-Diod e 102 provides emitter junction protection for trasistor 100 and speedslthe recovery of capacitor 96 and resis r 99'after occurenceofa high level signal.

Capacitor 104 and resistor 105 are chosen so that on time" of transistors and 106 is about 200 milliseconds. During that time, further pulses appearing at the base of transistor 100 have no effect on reset transistor 114 since transistor 100 is already turned on and neither a signal driving it on harder nor one tending to turn it off momentarily are felt by reset transistor 114.

When transistors 100 and 106 turn off, capacitor 104 begins to discharge through resistor 101, resistor .1107 and resistor 105. As soon as current flows through resistorfrom this discharge current, a forward bias is created onthe base of transistor 109- and capacitor-104 rapidly dumps through the collector-emitter circuitoftransistor 109. Thus, the refractory circuit has aquick recovery time.

As transistor 100 turns on, charge current flows through capacitor 110 and provides base current for reset transistor 114 until capacitor 110 becomes fully charged. Reset transistor 114 then turns on, shorting the collector of transistor 31 directly to ground conductor 19 and dumping timing capacitor 36. When transistor 100 turns off, capacitor 110 dischargesquickly through resistors 101, 112 and 111. Reset action occurs in response to either a sensed natural R- wave or to a generated pacer pulse. In either case, itresets timing capacitor 36 to a predetermined level and starts the timing cycle again in synchronization with the sensed signal.

The signal transit times in voltage increasing circuit 12, amplifier 15, refractory circuit 16, and reset circuit 17 and the fact that capacitor 36' isnormally dumped to a slightly different reference level by reset transistor 114, than it is by diode 43 and transistor 38 make it clear that the time between pulses in the pacer described normally is slightly different when the demand circuitry is disabled and when it is active. In the preferred form, the pulse rate scale is calibrated'when the demand circuitry is active, because that is the normal use mode.

it is obvious that transistors and field effect devices of the opposite conductivity type can be-used in-the invention by simply reversing battery polarities and making appropriate biasing adjustments.

lclaim:

l. A regulated power supply providing a current which is substantially independent of relatively large supply voltage variations; the supply comprising:

a source of voltage;

constant current means providing a substantially constant current independent of relatively wide variations in voltage thereacross; constant voltage means for providing a substantially constant voltage thereacross when a substantially constant current is applied thereto, serially connected to the means for providing a substantially constant current;

circuit means connecting the serial combination of the constant current means and the constant voltage means across the source of voltage;

current control means having a pair of input terminals and a pair of output terminals, exhibiting a high input impedance between the input terminals, providing a current between the output terminals which is substantially independent of source voltage variation when a relatively constant voltage is applied between the input terminals, and having the input terminals connected across the constant voltage means;

the constant current means comprising a field effect device having source, drain, and gate electrodes with thedrain electrode serially, connected to the constant voltage means, the gate electrode connected to the source of voltage, and impedance means connecting the source elec-- trode to the source of voltage;

the current control means including variable resistance means, a transistorhaving emitter, base and collector electrodes with the base electrode connected to one end of the constant voltage means the variable resistance means connected between the emitter electrode and the other end of the constant voltage'means, and the collector'electrode connected to one of the output terminals.

2. A cardiac pacer having a highly stable, controllable pulse generating rate comprising:

a power supply according to claim 3;

pulse generating means connected to the source of voltage,

the pulse generating means including current controlled timing means connected to at least one of the output ter minals of the current control means;

electrode means, including at least one electrode adapted to contact cardiac tissue, connected to the pulse generating means;

condition responsive means adapted to be responsive to the beating action of a patientsheart to provide a signal indicative of a heartbeat; and

reset means connected to receive signals from the condition responsive means and to reset the timing means to a predetermined level in response thereto.

3. A regulated power supply providing a controllable magnitude current which is substantially independent of relatively large supply voltage variations, the supply comprising:

a voltage source;

a field effect device. having source, drain, and gate electrodes;

first resistance means connected between the source electrode the gate electrode;

second resistance means;

circuit means serially connecting the first resistance means.

the source and drain electrodes, and the second resistance means across the voltage source, thereby providing a predetermined voltage drop across the second resistance means so long as substantially all the current flowing through the source-drain circuit of the field effect transistor, and no other current, flows through the second resistance means;

a transistor having emitter, base and collector electrodes, with the base electrode connected to one end of the second resistance means; and

variable resistance means connected between the emitter and the opposite end of the second resistance means for controlling the magnitude of the current at the collector.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION December 15, 1970 Patent No? 547 ,127 Dated Robert K. Anderson Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 41 "20 20" should read 20% Colux 2 line 26 "increases" should read increased Column line 6, "chose" should read chosen Column 6 line 67 insert capacitor 79 and that containing following 78 and,

"3" should read l resistor Claim 2, line 3,

Signed and sealed this 30th day of March 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, J Attesting Officer Commissioner of Patent 

